When using photovoltaic installations exhibiting solar modules of large dimensions, the variable position of the sun in the course of the day always gives rise to partial shadowings of the surfaces equipped with solar cells, which are caused for example by neighboring constructions, antennas, trees, etc. Even when the surface in shadow is a small area, these passing shadows may give rise to appreciable losses of power of the system as a whole consisting of several identical modules. On the one hand, the input of light is directly decreased in the region of shadow so that the relevant module creates less electric power. Moreover, the solar cells of a module are as a general rule connected electrically in series. If the current (variable as a function of incident light) in the module (or the module part) in shadow decreases, it also limits at the same time the current of the neighboring cells, not in shadow, of the same module. Furthermore, the current is likewise limited by other modules connected in series.
To limit these inevitable disturbances, each solar module or each overall system is divided into a plurality of partial systems (“networks”). Each of these is furnished with its own rectifier (“network rectifier”). For the sake of standardization, these rectifiers have a minimum power of around 700 watts. This corresponds to the power created by a photovoltaic arrangement of around 7 to 8 m2 in size.
In such an arrangement, a photovoltaic voltage which may reach around 500 to 600 volts is created and processed with modern electronic equipment and several modules connected in series, an individual solar cell having a working voltage of around 0.5 volts. Consequently, such a network can comprise around 1000 to 1200 individual solar cells. Strong local shadow on a small number of solar cells (1 to 5% of the total surface area) may then result in a loss of power of 75% over the system as a whole.
The known general technique for preventing the current from passing through solar cells in shadow or those which are damaged is by way of so-called shunt diodes, so that its decrease cannot act as strongly on the system as a whole. The shunt diode allows the flow of a short-circuit current when the internal resistance of the solar cell exceeds the voltage drop across the diode.
EP-0 896 737 B1 discloses a photovoltaic solar arrangement furnished with an integrated disconnection device which neutralizes the electric power of the module when it is activated by an external switching device. This device should moreover not limit the negative consequences of partial shadows, but render the relevant solar module inoperative if it has for example been dismantled in an unauthorized manner. Manipulations of this disconnection device are possible only after destruction of the module as a whole.
Solar elements are known (DE-A1-42 08 469), the solar cells of which serve as sensors for measuring the effective solar radiation. The value detected by these cells can for example be used to depict by means of display screens the instantaneous solar radiation on the relevant module.
U.S. Pat. No. 4,175,249 discloses an arrangement of photovoltaic solar cells with integrated control, in which an independent solar cell additional to a series of several current-creating identical solar cells is used merely as incident-light sensor. This solar cell serving as sensor is exposed to the same conditions of temperature and of light as the current-creating solar cells. Its no-load voltage is used as measurement signal, is amplified and compared with the instantaneous output voltage of the other cells. Depending on the result of this comparison, the solar cells of the arrangement can be connected automatically in various states of series and parallel connection by way of relays. On each occasion this should yield the maximum possible output charging voltage.
The invention relates to a process for managing a solar module as a function of incident light and, starting from an arrangement according to U.S. Pat. No. 4,175,249 mentioned above, to create a solar module that is improved from the standpoint of the effects of a partial shadow.